Nitrogen trifluoride (NF.sub.3) is a highly valued material and fluorine source that has found value in the electronics fabrication industry primarily as an etchant for the production of nano-scale geometries in electronic devices and as a cleaning medium for fabrication equipment. As with all electronic materials, the industry is driven to seek increasingly purer NF.sub.3 for use in its production facilities. For some applications, purities as high as 99.999% NF.sub.3 are desired.
Carbon tetrafluoride (CF.sub.4) is a typical contaminant found in many synthetic routes to the production of NF.sub.3. The removal of CF.sub.4 from NF.sub.3 by conventional separation methods, such as distillation and bulk absorption, is not possible due to the similar boiling points, molecular sizes and heats of absorption of CF.sub.4 and NF.sub.3. The contamination of NF.sub.3 with CF.sub.4 is a particular problem for the electronics industry due to the potential for carbon deposition in the nano-scale geometries of electronic devices.
Purification techniques for NF.sub.3 are known in the prior art. For instance, U.S. Pat. No. 4,156,598 discloses a technique for removing N.sub.2 F.sub.2 from NF.sub.3 by contacting the NF.sub.3 and N.sub.2 F.sub.2 containing gas with a nickel catalyst and subsequent adsorption of N.sub.2 F.sub.2 by-products on zeolitic adsorbents. Removal of CF.sub.4 is not discussed. U.S. Pat. No. 4,193,976 discloses a defluorination process for purifying NF.sub.3 of N.sub.2 F.sub.2 using nickel catalysts. The presence of CF.sub.4 is identified at Col. 4, line 58, and Example 3 suggests that the use of 5A zeolitic molecular sieve is inappropriate as a purification agent due to the high degradation of the desired NF.sub.3.
In light of the close boiling points of NF.sub.3 (-129.degree. C.) and CF.sub.4 (-128.degree. C.), distillation techniques for bulk recovery of high purity NF.sub.3 from CF.sub.4 are not viable. Distillation of NF.sub.3 from other compounds is readily recognized, such as demonstrated in U.S. Pat. No. 3,181,305 wherein NF.sub.3 is distilled from nitrous oxide and tetrafluorohydrazine. In addition, the dipole moments and the heats of adsorption of NF.sub.3 and CF.sub.4 are sufficiently close so that bulk recovery of NF.sub.3 from conventional bulk adsorption technologies would not be feasible.
Analytical gas chromatography for the determination of NF.sub.3 among other fluorine-containing compounds has been disclosed in an article "Analysis of F.sub.2, HF, NF.sub.3, t-N.sub.2 F.sub.2, and N.sub.2 F.sub.4 Mixtures by Gas Chromatography" by Larry G. Spears, et al., appearing in the Journal of Gas Chromatography, Vol. 6, July 1968, pp. 392 and 393.
U.S. Pat. No. 4,772,296 discloses a preparative gas chromatography technique for preparing electronic industry level purities of various Group 3 and Group 5 elements and their halides. Nitrogen halides are contemplated. The separation is performed on a chromatographic column packed with porous polymer such as "Poropak".
More specifically, U.S. Pat. No. 3,125,425 discloses that CF.sub.4 may be separated from NF.sub.3 by gas chromatography in which a perhalogenated polymer is coated on silica gel as the chromatographic adsorption medium. This constitutes gas-liquid equilibrium chromatography, wherein the gas is the CF.sub.4 and NF.sub.3 and the liquid is the polymer coating on the silica gel support. High speeds of operation are specifically disclosed at Col. 4, lines 19 through 21.
These chromatographic techniques operate on an equilibrium basis. In an equilibrium chromatographic separation, the absorbent over the length of the chromatographic bed acts as an infinite series of distillation trays or adsorption sites at which the various components of the gas to be resolved equilibrate with regard to their individual adsorption characteristics. The chromatographic process carrier gas sweeps the gas species of the feed gas over the length of the adsorbent in the chromatographic column to present the gas elements to each stage of the adsorbent and the chromatographic column. However, under equilibrium conditions of operation, such chromatographic separations will be difficult in producing bulk or volumetric quantities of the gas species to be separated if the heats of adsorption of individual gas species are closely matched, resulting in poor resolutions, poor purities, slow processing times and/or significant sizing of equipment for bulk high purity separations.
The equilibrium nature of preparative gas chromatography is substantiated by the literature reference "Gas Separation By Adsorption Processes" by Ralph T. Yang at page 213, wherein it is stated, "The high separating efficiency of chromatography as compared to adsorption processes is caused by the continuous contact and equilibration between the gas and sorbent phases. Each contact is equivalent to an equilibrium stage or theoretical plate." At page 214 of the same literature, it is stated, "In the plate theory, the column is thought to consist of a finite number of hypothetical stages, and equilibrium is attained in each stage."
Additionally, in the literature reference titled, "Principles of Adsorption and Adsorption Processes," by Douglas M. Ruthven, in Chapter 10 on "Chromatographic Separation Processes," at page 325, it is stated, "The separating power of a chromatographic process arises because during passage through the column, each molecule of an adsorable species is equilibrated many times between the mobile and stationary phases. Each equilibration is equivalent to one `equilibrium stage` or `theoretical plate`."
Canadian Patent 785,747 discloses hydrothermally treated Na A zeolite to remove water from freons.
Hydrothermal stability of A-type zeolites was reported on in the literature, but the materials had no applicability when reproduced for use by the present inventors for the present invention, E. G. Tochilorskaya et al; Grozen. Neft. Nausch, Issled. Inst., No. 25,82-90.
Diffusion studies on 4A and 5A zeolites have been conducted regarding adsorbed molecules, J. Karger et al., Zeolites, Vol. 9, (1989) pp 267-281; J. Karger et al., Zeolites, Vol. 6, (1986) pp 146-150; J. Karger et al., Zeolites, Vol. 7 (1987) pp. 90-107.
Structural changes during pore closure of zeolites have been studied for 4A and 5A zeolites as reported in J. Karger et al., Zeolites, Vol. 7 (1987) pp. 282-285.
European Appln. 0 366 078 discloses the purification of NF.sub.3 from N.sub.2 O, CO.sub.2 and N.sub.2 F.sub.2 using thermally treated zeolites. The process is an equilibrium adsorption, and no restriction is placed upon the zeolitic pore size.
The prior art demonstrates that various techniques for NF.sub.3 purification have been attempted and that CF.sub.4 is a known contaminant of NF.sub.3, which has been traditionally resolved using equilibrium based chromatography and, more specifically, by equilibrium based gas-liquid chromatography. Such equilibrium based separations suffer from disadvantages of efficiency when bulk or volume purifications to high purity levels are desired. The present invention overcomes these drawbacks of the prior art as will be set forth in greater detail below.